The present invention relates to optical devices such as light guides. More particularly, the present invention relates to light guides for use with backlit displays.
A variety of devices have been proposed for illuminating electronic displays. These devices include backlighting panels, front lighting panels, concentrators, reflectors, structured-surface films and other optical devices for redirecting, collimating, distributing or otherwise manipulating light.
Efficient use of the light is particularly important in battery powered electronic displays such as those used in cell phones, personal digital assistants and laptop computers. By improving lighting efficiency, battery life can be increased and/or battery sizes can be reduced. Prismatic films are commonly used improve lighting efficiency and enhance the apparent brightness of a backlit liquid crystal display.
Display quality is also an important consideration in electronic displays. One measure of display quality for a backlit display is brightness uniformity. Because displays are typically studied closely or used for extended periods of time, relatively small differences in the brightness can easily be perceived. These types of variances in brightness can be distracting or annoying to a user. To soften or mask non-uniformities, a light scattering element (e.g., a diffuser) can sometimes be used. However, such scattering elements can negatively affect the overall brightness of a display.
Although a number of backlight designs have been proposed, there is an ongoing need for more efficient designs and for reduced power consumption. The present invention relates generally to backlit displays that efficiently use light and concurrently are capable of providing high levels of brightness uniformity.
One aspect of the present invention relates to a light guide including a light guide body having a light re-directing side positioned opposite from a light output side. A plurality of elongate prisms are located adjacent the light output side of the light guide body. The prisms extend side-by-side relative to one another along lengths. The prisms include upper edges defining heights of the prisms. The upper edges extend along the lengths of the prisms. Each upper edge includes a plurality of edge segments having different magnitudes of slope. The light guide also includes a plurality of facets located adjacent the light re-directing side of the light guide body for reflecting light toward the light output side. Generally flat surfaces are located between the facets for reflecting light along the length of the light guide by total internal reflection. In certain embodiments, the light re-directing side of the light guide can include recessed lands positioned between plateau segments. In such embodiments, first and second facets extend from the plateau segments to the lands.
Another aspect of the present invention relates to a backlit display having a light source and a light gating device (e.g., a liquid crystal display (LCD)). A light guide as described above is positioned behind the light gating device for directing light from the light source to the light gating device. In alternate embodiments, one or more optical elements (e.g., transmissive diffusers, polarizing films, reflective polarizing films, prismatic brightness enhancing films, etc.) can be positioned between the light guide and the light gating device. In other embodiments, a reflective element (e.g., a reflective diffuser, a polarization converter, etc.) can be positioned behind the light guide.
The various aspects of the present invention have particular applicability to recycling backlit displays. A typical recycling backlit display includes a light guide, a reflector positioned behind the light guide, and one or more light recycling structures positioned in front of the light guide. The light recycling structures reflect light of a selected type (e.g., light that does not have a selected polarization or is outside a predetermined viewing angle) back into the light guide to allow the reflected light to be re-used. The various aspects of the present invention also are applicable to one-pass backlit displays.